Batteries based on lithium (Li), such as lithium-ion batteries, are attractive due to their high energy density compared to other commercial batteries (e.g., alkaline batteries). Li-ion batteries are used commercially in computers, cell phones, and related devices. Li-ion batteries have potential for use in electric vehicle/hybrid-electric vehicle (EV/HEV) applications. The most-suitable battery technology, which offers both a sufficient range and enough power to provide the acceleration required by today's drivers, is the lithium-ion battery system (Grove and Burgleman, The McKinsey Quarterly, December 2008). However, safety requirements currently dictate the use of low-voltage Li-ion batteries. The major safety concern is flammability, which poses a serious problem today.
In a typical Li-ion battery, the Li+ cation moves from the anode to the cathode (discharging) or cathode to anode (charging). The electrolyte is a lithium salt in an organic solvent. Common carbonate-based electrolytes are flammable and may lead to fire in cases such as short circuiting and/or overcharging of the battery (S. Tobishima and J. Yamaki, J. Power Sources, 81-82, 882, 1999). Thermal decomposition of the carbonate solvents in the electrolyte and the metal-oxide cathode leads to generation of hydrogen radicals and oxygen, respectively. When hydrogen radicals and oxygen react, a chain reaction can generate a large amount of heat in the battery.
Fire-retardant materials have been studied and proposed for Li-ion batteries. Fire-retardant materials should reduce the rate of chain reactions of hydrogen and oxygen or react with Li ions to form stable compounds, thereby preventing thermal runaway in the battery.
Direct incorporation of a fire-retardant material as an additive or cosolvent into an electrolyte system leads to battery performance reduction. For example, trimethyl phosphate (TMP) has been found to be an effective fire-retardant material in a Li-ion battery. High TMP/cosolvent concentrations can effectively suppress the flammability, but at the expense of battery performance. TMP-related performance reduction results from electrochemical instability of phosphates causing severe capacity fading, as well as high viscosity of cosolvents which reduces capacity utilization and power.
One of the significant efforts to improve the safety concern of the lithium-ion batteries is to reformulate electrolytes by employing flame retardants to suppress the fire and explosion tendencies of the lithium-ion batteries (C. W. Lee et al., Electrochem. Solid-State Lett., 3, 63, 2000; S. C. Narang et al., U.S. Pat. No. 5,830,660, 1998; X. M. Wang et al., J. Electrochem. Soc., 148, A1058, 2001; X. M. Wang et al., J. Electrochem. Soc., 148, A1066, 2001). The non-flammability of TMP was explained by a flame-retarding mechanism involving a hydrogen radical trap in the gas phase (Z. E. Jolles, Plast. Polym., 40, 319, 1971; W. G. Schmidt, Trans. J. Plastics Inst., 1965, 248).
Wang et al. (Xianming Wang et al., Journal of The Electrochemical Society, 148, 10, A1058-A1065, 2001) found that the TMP solvent had good oxidation stability and poor reduction stability, which led to TMP reduction decomposition on the natural graphite electrode at the negative potential of 1.2 V. To solve this problem, ethylene carbonate, propylene carbonate, and diethyl carbonate cosolvents were mixed with TMP solvent. The authors claimed that the reduction decomposition of the TMP solvent was considerably suppressed in TMP containing these three cosolvents. The performance improvement was attributed to the formation of good solid-electrolyte interphase film on natural graphite electrode in the mixed electrolyte.
Xu et al., Journal of The Electrochemical Society, 149, 5, A622-A626, 2002, studied effects of using alkyl phosphates and a cyclophosphazene as cosolvents on the non-flammability of TMP-based electrolytes for lithium-ion batteries. It was found that although the cosolvents at high contents (10%) effectively suppress the flammability of the electrolyte, their flame-retarding effectiveness is still insufficient to render the electrolytes completely nonflammable. Furthermore, such reduction in electrolyte flammability is always realized at the expense of performance, according to this publication.
Xu et al., Journal of The Electrochemical Society, 150 (2), A161-A169, 2003, studied fluorinated versions of phosphates as modified flame retardants. It is known that both phosphorous and flourine are effective flame-retarding elements. The authors hoped that such combinations of these elements into a single molecule would work more effectively than they would separately. It was described therein that the addition of these fluorinated phosphates to electrolyte solutions reduced the overall flammability at the expense of ion conduction, while electrochemical stability on carbonaceous anodes was improved.
So far, attempts to formulate an electrolyte based on TMP that is non-flammable and also works well in lithium-ion cells has not been very successful. There always has been a trade-off between electrolyte flammability and its performance either due to the electrochemical instability on graphitic anode, e.g., triethyl phosphate (TEP) and trimethyl phosphate (TMP), or due to other adverse properties of the fire-retardant materials, such as high melting point and high viscosity.
Due to reduced performance, flame retardants reported in the prior art are normally used only at very low concentrations, usually about 5% or less, an amount that is insufficient to render the system non-flammable (X. M. Wang, et al., J. Electrochem. Soc., 148, A1058, 2001; X. M. Wang, et al., J. Electrochem. Soc., 148, A1066, 2001; C. W. Lee et al., Electrochem. Solid-State Lett., 3, 63˜2000). Low efficiency of these materials in flame retarding further worsens the trade-off, because as higher flame-retardant content is needed in the electrolyte to reduce the flammability to a desired level, the negative impact on performance increases proportionally with flame-retardant content.
In view of the significant aforementioned shortcomings described with respect to the prior art, methods and systems are needed to provide high-voltage batteries (e.g., high-voltage Li-ion batteries) while meeting safety requirements due to flammability. Specifically, what are needed are methods and systems that can minimize capacity fading, increase capacity utilization, and maintain power during use.